Polylactic acid (PLA or polylactide) is a polymer having optical character, and is used in sutures for surgery, microcapsules for injection, and other biomedical materials for biodegradation, and is a biodegradable eco-friendly material which can be used in packing materials, electronic appliances, office supplies, vehicle interior materials, and various polymer products.
In order for the polylactic acid to be used for the above purposes, a high optical purity (D-type or L-type optical isomers) and a high molecular weight are required, which requires a high optical purity and a chemical purity of lactide, a monomer used in a preparation of polylactide.
A traditional method of preparing lactide from lactic acid goes through a two-step reaction of primarily polymerizing lactic acid under reduced pressure to obtain a prepolymer with a molecular weight of 500 to 5000, and depolymerizing the prepolymer under inactive gas flow and reduced pressure, thereby obtaining lactide, as shown in Route (1) of Reaction 1 (U.S. Pat. Nos. 5,274,073, 5,247,059, 5,274,127, and 6,277,951). Commercially, Nature Works in the U.S. has been producing lactide from L-type lactic acid raw material through the two-step reaction and polycondensation of the lactide in a polymerization reactor to produce about 140,000 tons of L-type PLA bioplastics annually. Also, Purac in the Netherlands has recently built factories for the preparation process of lactide by the two-step reaction in Thailand and initiated commercial production thereof.
However, using the two-step reaction may result in deterioration of the prepolymer and the lactide products in the reactor over a long period of time, causing a problem of increased production of a byproduct, meso-lactide, during production of L- or D-lactide. The two-step process of lactide production by prepolymerization-depolymerization has problems in that a prepolymer oligomer, which is a reactant in the depolymerization step, is not degraded into lactide, some oligomers are over-polymerized, and waste mixed with catalysts contained in the prepolymer is produced. Additionally, the two-step reaction process requires a vacuum pump for reducing high vacuum pressure, and the reaction apparatus is very complex and excessively costly.
Meanwhile, as another traditional method for preparing lactide from lactic acid, there exists a method which directly prepares lactide via a fixed bed reactor filled with solid acid catalysts by evaporating lactic acid at a temperature at 200° C. or higher without going through a prepolymer, as shown in Route (2) of Reaction 1 (U.S. Pat. No. 5,332,839). In particular, the present inventors suggested a direct preparation method of lactide using a nanocomposite catalyst comprising tin oxide as a main component to prepare lactide from an aqueous solution of lactic acid, and this method enables a stable production of lactide with a high yield of 90% or higher (Korean Patent No. 1376483).

In the case of using a nanocomposite catalyst comprising tin oxide as a main component, the lactide produced after the catalytic reaction is present in the form of vapor along with water, unreacted lactic acid, and carrier gases, which are byproducts of the reaction. Meanwhile, as lactide readily reacts with water and lactic acid and denatures to a lactic acid dimer or trimer, the lactide which is produced after the catalytic reaction needs to be rapidly separated from water and lactic acid.
As one of purification and separation methods, a method of separating meso-lactide using the differences in the solubility in water and the rate of hydrolysis between the meso-lactide and D- and L-lactide has been extensively studied (Reference: U.S. Pat. No. 5,502,215). However, such method is disadvantageous in that the separation is not efficient as it is a technique of merely stirring lactide and water at room temperature. Additionally, the lactide is dissolved in a solvent immiscible with water to be in contact with water to eliminate components through extraction. However, such method does not sufficiently separate L- or D-lactide from meso-lactide.
In Korean Patent No. 10-0261850 (Title of Invention: Preparation method of polylactide using improved purification process, hereinafter referred to as Conventional Technology 1), as a preparation method for polylactide comprising providing a crude lactide mixture comprising at least one lactide selected from the group consisting of meso-, L-, and D-lactide, a preparation method for polylactide comprising distilling the crude lactide mixture without extracting solvent or recrystallizing to purify the mixture as at least one lactide fraction; and polymerizing the purified lactide fraction to form polylactide is disclosed.